This is a battery powered motor for a small rowing boat. The motor used is from an electric wheelchair which ran from 24V. The motor has a built in reduction gearbox to provide a large torque to the wheels on the wheelchair. This gearing ratio was far to high for the propeller. The prop needs to turn much faster in order to generate the right amount of thrust to move the boat.
Fortunately the motor used had a small part of the main rotor protruding from the opposite end to the gearbox which was sufficient to attach a sprocket. This sprocket was linked with a chain to another sprocket on the main axle. This axle was then connected to a 90 degree 2:1 gearbox from a cheap angle grinder which was then attached to the propeller.
The propeller is from an old barge and is a little larger than necessary, but the motor provides just enough power to compensate.
The control system uses a pulse width modulation system for a smooth speed control and efficiency. The circuit used is based on the power pulse modulator but with the high voltage MOSFET being replaced by an array of lower voltage high current bipolar transistors so that the huge currents demanded by the motor could be tolerated.
The other switches on the control box are for forward/stop/reverse and to choose between 12V and 24V operation. These are simply large power switches that are used to directly switch the power as necessary.